Convection is the transfer of heat by circulating it through air or liquids. Unlike conduction, in which there must be direct or indirect contact between the two objects for heat transfer to take place, convection relies on the circulating motion of the molecules in order to transfer heat. Also unlike conduction, which relies on microscopic movement of particles to transfer heat, convection is a bulk transfer of a lot of mass at the same time.
Like conduction, though, the heat transfer in convection moves from the hotter area to the colder one.
1. Heating Water on the Stove
When a pot of water is placed on a stove and the stove is turned on, the pan itself becomes increasingly hotter due to conduction; that's where the metal of the pot is directly in contact with the heating element. But the water inside the pot is heated due to convection. As the water at the bottom of the pot (touching the increasingly hotter metal) rises, it transfers heat to the water above it. The cold water is pushed downward towards the hot bottom of the pot due to convection currents, and the process continues.
2. A Hot Air Balloon
Hot air balloons rise due to the propensity of warmer air to be less dense than the air around it. A heat source at the bottom of the balloon heats the air molecules around the flame, and those molecules rise. Warmer air is less dense than cold air, so as the warm air rises the molecules spread out. The cold air is pushed downward, where it is also heated. The swirling movement of the warmer air as it rises continues to increase the temperature of the air around it.
3. Warm Weather and Bodies of Water
Weather is largely affected by convection, as air creates breezes over land masses located next to large bodies of water like lakes or oceans. Water has a higher heat capacity than earth, so it holds its heat better. That means it also takes longer to change the water's temperature in either direction. At daytime, the air over the body of water will be a lower temperature than the air over land, creating a low pressure area over the land and a higher pressure area over the water. This movement of air molecules from one pressure system and temperature to the other causes breezes to blow from water to land, altering the temperature. The opposite scenario occurs at night when the sun goes down and the water cools off more slowly than the land.
4. Dinner Preparation
If meat is still frozen when it's time to start cooking, it will thaw more quickly when placed under running water than if it is immersed in water. The reason is the convection, or movement of the water and its heat circulation, will transfer heat more quickly into the frozen meat than if the meat sits immersed in water and has to absorb heat energy through conduction.
5. The Earth's Convection
The Earth's mantle moves very slowly due to the convection currents beneath the surface. These currents transfer heat from the Earth's hot core, sending them up to the surface. The swirling currents cause the tectonic plates to move very gradually around the planet's surface. At the same time, new hot matteradheres to the growing edges of the plates, and then it cools. The material becomes denserwhen the heat causes it to contract and sink back into the mantle at an ocean trench, causing the formation of a volcano.